1. Pair production of scalar top quarks in polarized gamma-gamma collision at ILC
A.Bartl (Univ. of Vienna)
W.Majerotto HEPHY (Vienna)
K.Moenig DESY (Zeuthen)
A.N.Skachkova, N.B.Skachkov
JINR (Dubna)

2. The Photon beams at ILC
The option of a photon collider at ILC will be achieved by using backscattered photon beams by Compton scattering of laser beams on the electron beams.
Unlike the situation at e+e- collider, the energy of the beams of backscattered photons will vary from event to event.
Circe2 program gives the energy spectra of the colliding backscattered photons & the values of photon beam luminosities.
QUARKS’2010 A.N.Skachkova. “Stop pair production in polarized γγ collisions at ILC”

3. Photon beam characteristics
The monochromaticity of the backscattered photon beams is considerably increased if the mean helicities λe and Pc of the electron beam and laser photon beam are chosen such that 2λePc ≈ -1.
In this case the relative number of hard photons becomes nearly twice as large in the region of the photon energy fraction Y= Eiγ / Eebeam ≈ (i=1,2) and the luminosity in collisions of these photons increases by a factor of 3-4.
QUARKS’2010 A.N.Skachkova. “Stop pair production in polarized γγ collisions at ILC”

4. Energy spectrum from CIRCE 2 shows the degree of monochromaticity of the backscattered photons
We used as a reasonable approximation the CIRCE2 output spectra generated for
E tot e+e- = 800 GeV and scaled them (by 1000/800) to the higher beam energy
2Eebeam = 1000 GeV
The left peak is caused by multiple Compton scattering and beamstrahlung photons
The right one at the energy fraction
Y = Eiγ / Eebeam ≈ 0.83 (i=1,2)
is due to the hard photon production
Only on a 0.3% the γγ energy is high enough for γγ  stop stop bar production
QUARKS’2010 A.N.Skachkova. “Stop pair production in polarized γγ collisions at ILC”

5. γ-energy correlation Y1/Y2 spectra for J=0 enhanced (J – total angular momentum)
Whole γ spectrum
γ spectrum above
stop pair production
threshold
Polarizations + - /- +
Polarizations + +/- -
QUARKS’2010 A.N.Skachkova. “Stop pair production in polarized γγ collisions at ILC”

6. γ-energy correlation Y1/Y2 spectra for J=2 enhanced
Whole γ spectrum
γ spectrum
above
stop pair production
threshold
Polarizations + - /- +
Polarizations + + /- -
QUARKS’2010 A.N.Skachkova. “Stop pair production in polarized γγ collisions at ILC”
QUARKS’2010 A.N.Skachkova. “Stop pair production in polarized γγ collisions at ILC”

7. STOP pair production cross sections
e+ e- CM energy = 1000 GeV
STOP pair production cross sections
Ϭ = fb “+ -” & “- +”
Ϭ = fb “+ +” & “- -”
4-6 factor difference
TOP pair production
cross sections
Ϭ = fb “+ -” & “- +”
Ϭ = fb “+ +” & “- -”
PYTHIA cross section distribution formula taken from S.Berge et al. hep-ph/
QUARKS’2010 A.N.Skachkova. “Stop pair production in polarized γγ collisions at ILC”
QUARKS’2010 A.N.Skachkova. “Stop pair production in polarized γγ collisions at ILC”
QUARKS’2010 A.N.Skachkova. “Stop pair production in polarized γγ collisions at ILC”
QUARKS’2010 A.N.Skachkova. “Stop pair production in polarized γγ collisions at ILC”

8. ( b & bbar - jets, 2 jets from W  qi qj decay and μˉ ).
The subsequent decay channels have been considered:
STOP STOP → b χ1+ b bar χ1- → b b bar qi qj bar μ- νμ χ1º χ1º
t t → b W+ b bar W- → b b bar qi qj bar μˉ ν μ
The only difference of STOP / TOP production is the presence of the
two non-detectable neutralinos in the case of stop pair production.
The quarks hadronize into jets. Jets are determined by use of PYCLUS jetfinder based on “Durham” cluster distance measure algorithm.
Both the signal and background events have the same experimental signature
( b & bbar - jets, 2 jets from W  qi qj decay and μˉ ).
QUARKS’2010 A.N.Skachkova. “Stop pair production in polarized γγ collisions at ILC”

9. Cross-section σ dependence on Mstop
QUARKS’2010 A.N.Skachkova. “Stop pair production in polarized γγ collisions at ILC”
QUARKS’2010 A.N.Skachkova. “Stop pair production in polarized γγ collisions at ILC”

10. In order to simulate the STOP pair production, we assumed the
following scenario for the MSSM model parameters:
M ~Q = M ~t L = 270 GeV (left squark mass)
M~U = M ~t R = 270 GeV (right squark mass)
At = -500 GeV (top and bottom trilinear coupling)
μ = GeV
tanβ = 5
M1 = 80 GeV
M2 = 160 GeV
Our aim is:
To find out physical variables (Energy, PT, angle and invariant mass distributions) most suitable for signal (stop) / background (top) separation
To estimate the corresponding values of cuts on these variables
Corresponds to
Mstop = GeV,
M χ1º = GeV
M χ 1+ = GeV
QUARKS’2010 A.N.Skachkova. “Stop pair production in polarized γγ collisions at ILC”

11. STOP Energy distributions
The shapes of these spectra follow backscattered γ-energy distributions
Polarization : ++/ --
Polarization : +- / -+
QUARKS’2010 A.N.Skachkova. “Stop pair production in polarized γγ collisions at ILC”

12. STOP PT distributions
Polarization : +- / -+
Polarization : ++/ --
The PT-spectrum for “++/--” polarization is much softer then for “+-/-+” polarization
QUARKS’2010 A.N.Skachkova. “Stop pair production in polarized γγ collisions at ILC”

13. STOP angle Θ distributions
Polarization : +- / -+
Polarization : ++/ --
QUARKS’2010 A.N.Skachkova. “Stop pair production in polarized γγ collisions at ILC”

14. Minv (stop+stopbar) = Minv (γγ) = √ (Pγ1 + Pγ2)2 distributions
The shapes of these spectra also follow backscattered γ-energy distributions
Polarization : +- / -+
Polarization : ++/ --
QUARKS’2010 A.N.Skachkova. “Stop pair production in polarized γγ collisions at ILC”

15. E- spectra of jets from W
STOP
TOP
Polarization : +- / -+
Polarization : ++/ --
QUARKS’2010 A.N.Skachkova. “Stop pair production in polarized γγ collisions at ILC”

16. PT- spectra of jets from W
STOP
TOP
Polarization : +- / -+
Polarization : ++/ --
QUARKS’2010 A.N.Skachkova. “Stop pair production in polarized γγ collisions at ILC”

17. W mass reconstruction as Minv of 2 Wjets
STOP
Clear seen visible virtual nature of W boson
TOP
Partonic level
Level of jets
QUARKS’2010 A.N.Skachkova. “Stop pair production in polarized γγ collisions at ILC”

18. E- spectra of b-jets (B-jet is determined as a jet that includes b-meson)
STOP
TOP
Polarization : +- / -+
Polarization : ++/ --
QUARKS’2010 A.N.Skachkova. “Stop pair production in polarized γγ collisions at ILC”

19. PT- spectra of b-jets STOP TOP Polarization : +- / -+
QUARKS’2010 A.N.Skachkova. “Stop pair production in polarized γγ collisions at ILC”

20. Cos (b, bbar) spectra for Stop pairs production
Polarization : +- / -+
Polarization : ++ / --
Most of b- and b bar – jets move approximately in the opposite directions,
but some are in the same hemisphere
QUARKS’2010 A.N.Skachkova. “Stop pair production in polarized γγ collisions at ILC”

21. μ distributions in the signal events
PTμ
Signal μ’s
Fake μ’s
QUARKS’2010 A.N.Skachkova. “Stop pair production in polarized γγ collisions at ILC”

22. Missing energy (νμ , ~χ1º , beam pipe) distributions
STOP
TOP
Polarization : +- / -+
Polarization : ++ / --
QUARKS’2010 A.N.Skachkova. “Stop pair production in polarized γγ collisions at ILC”

23. Total scalar Σni=1 |PTi| variable
A cut PT skalsum < 180 GeV would lead to a good Signal / Background separation
STOP
TOP
Polarization : +- / -+
Polarization : ++/ - -
QUARKS’2010 A.N.Skachkova. “Stop pair production in polarized γγ collisions at ILC”

24. Missing mass Mmissing = √(√s-(ΣNjet n=1Enjet+Eμ))2-(ΣNjet n=1Pnjet+Pμ))2
STOP
Good for Signal / Background separation !
Mmissing > 650 GeV
TOP
Polarization : +- / -+
Polarization : ++/ --
QUARKS’2010 A.N.Skachkova. “Stop pair production in polarized γγ collisions at ILC”

25. Invariant mass of 4jets + μ
Good for Signal / Background separation with a cut < 230 GeV!
STOP
TOP
Polarization : +- / -+
Polarization : ++ / --
QUARKS’2010 A.N.Skachkova. “Stop pair production in polarized γγ collisions at ILC”

26. Invariant mass of 4 jets Minv(All jets)=√(Σi=1,2,3,4iPijet)2
Perfect for Signal / Background separation with cut M inv < 180 GeV!
STOP
TOP
Polarization : +- / -+
Polarization : ++ / --
QUARKS’2010 A.N.Skachkova. “Stop pair production in polarized γγ collisions at ILC”

27. Detected (visible) energy distributions
Perfect for Signal / Background separation with a cut E vis_tot < 250 GeV !
STOP
TOP
Polarization : +- / -+
Polarization : ++ / --
QUARKS’2010 A.N.Skachkova. “Stop pair production in polarized γγ collisions at ILC”

28. Used cuts for S/B separation
Polarization : +- / -+
Polarization : ++ / --
1. ) The events with clear recognized 2 B-jets (according to PYTHIA)
Stop cut efficiency = Stop cut efficiency = 0.80
Top cut efficiency = Top cut efficiency = 0.94
But, in the experiment inly 50% efficiency of the B-jets and B bar-jets separation and the 80% of the corresponding purity is expected
2. ) Invariant mass of all jets M inv(All jets) < 180 GeV
together with the cut above
Stop cut efficiency = Stop cut efficiency = 0.92
Top cut efficiency = Top cut efficiency = 0.008
3. ) Visible energy < 250 GeV
Stop cut efficiency = Stop cut efficiency = 0.98
Top cut efficiency = Top cut efficiency = 0.175
QUARKS’2010 A.N.Skachkova. “Stop pair production in polarized γγ collisions at ILC”

29. Final result of S/B separation
Polarization : +- / -+
Polarization : ++ / --
Achieved S/B ratio
With a loss of signal events
23 % %
The rest of Background events per year is only
while the estimated rate of Signal events per year
QUARKS’2010 A.N.Skachkova. “Stop pair production in polarized γγ collisions at ILC”

30. Invariant mass of Bjet & 2jetsW
Can also be used for Signal / Background separation with a cut < 120 GeV!
STOP
TOP
Polarization : +- / -+
Polarization : ++ / --
QUARKS’2010 A.N.Skachkova. “Stop pair production in polarized γγ collisions at ILC”

31. The most important variable - invariant mass of Bjet & 2jetsW
In the case of TOP pair production it gives
The mass reconstruction of the
top-quark M Top (175 GeV) :
M inv ( Bjet & 2jetsW ) = M Top
QUARKS’2010 A.N.Skachkova. “Stop pair production in polarized γγ collisions at ILC”

32. Stop invariant mass M inv (STOP) = M χ1º + M inv (bjet , 2jetsW) =
The reconstruction of the STOP
invariant mass M STOP (167.9 GeV):
M inv (STOP) =
M χ1º + M inv (bjet , 2jetsW) =
= M χ1º + √ (Pbjet + Pjet1 W +Pjet2W) 2
But χ1º - is not detectable particle
QUARKS’2010 A.N.Skachkova. “Stop pair production in polarized γγ collisions at ILC”

33. Invariant mass of Bjet & 2jetsW
For the case of STOP pair production gives
M inv (Bjet , 2jetsW) = M inv (STOP) - M χ1º
Right edge of Minv (Bjet , 2jetW) ≈ 87 GeV
M χ1º ≈ 80 GeV
Mstop = M χ1º + M inv(Bjet , 2jetW) =
= 167 GeV
QUARKS’2010 A.N.Skachkova. “Stop pair production in polarized γγ collisions at ILC”
QUARKS’2010 A.N.Skachkova. “Stop pair production in polarized γγ collisions at ILC”

34. The test of the other Scalar top mass
M stop = GeV
M χ1º = GeV
M χ1+ = GeV
Right edge of the peak of Minv (bjet , 2jetW)
≈ 120 GeV
M χ1º ≈ 80 GeV
Mstop = M χ1º + M inv(bjet , 2jetW) = 200 GeV
Polarization : +- / /--
Events/year :
QUARKS’2010 A.N.Skachkova. “Stop pair production in polarized γγ collisions at ILC”

35. Conclusion The main results:
1. New code for cross section of STOP pairs production that allows to take into account the polarizations of colliding photons is implemented into PYTHIA 6.
An account of the energy spectrum of colliding photons is done by use of CIRCE 2.
2. It is shown also that the invariant mass of the final jets and the visible energy variables turns out to be most efficient for signal / background separation.
3. A possibility of a good MSTOP reconstruction from right-hand edge point of
3 jets ( Bjet + 2 jetsW ) is demonstrated.
So, finally, it is shown that in a region of small values of stop mass ~ 167 GeV
the channel
STOP STOP → b χ1+ b χ1- → b b q q’ μ- νμ χ1º χ1º
is very promising for the STOP-quark search!
QUARKS’2010 A.N.Skachkova. “Stop pair production in polarized γγ collisions at ILC”

36. Outback
QUARKS’2010 A.N.Skachkova. “Stop pair production in polarized γγ collisions at ILC”

37. Publications & previous presentations
“Pair production of scalar top quarks in polarized photon-photon collisions at ILC. “
Authors: A.Bartl, W.Majerotto, K.Mönig, A.N.Skachkova, N.B.Skachkov
arXiv: , ILC-NOTE
“ Stop pair production in polarized photon-photon collisions”.
A.Bartl, W.Majerotto, K.Moenig, A.Skachkova, N.Skachkov.
Talk at the “International Conference on Linear Colliders”, LCWS04, 19-23 April 2004 , Paris, France
Proceedings of the “International Conference on Linear Colliders”; LCWS 04 , volume II, p
“Stop pair production at ILC” (
Talk at the International Linear Collider Workshop (14-17 November 2005, Vienna, Austria)
“Stop pair production in photon-photon collisions at ILC”
Десятая научной конференции молодых ученых и специалистов ОИЯИ. Дубна, 6-10 февраля 2006 г.
Труды десятой научной конференции молодых ученых и специалистов ОИЯИ. Дубна, 6-10 февраля 2006 г. стр , ISBN
A.N.Skachkova. “Stop pair production in polarized gamma-gamma collisions at ILC”

38. Publications & previous presentations
“Pair production of scalar top quarks in e+e- collisions at ILC. “
Authors: A.Bartl, W.Majerotto, K.Möniq, A.N.Skachkova, N.B.Skachkov
arXiv: , ILC-NOTE
“On pair production of scalar top quarks in e+e- collisions at ILC and a possibility of their mass reconstruction”
Authors: A.Bartl, W.Majerotto, K.Moenig, A.N.Skachkova, N.B.Skachkov
arXiv: , Phys.Part.Nucl.Lett.6: ,2009
Talk at the “QUARKS-2008” (
15th International Seminar on High Energy Physics Sergiev Posad, Russia, May, 2008.
Published in the proceedings
“Hadronic jets and search for stop quarks at ILC. “
Talk XIX International Baldin Seminar ISHEPP’08 (
XIX International Baldin Seminar on High Energy physics problems “Relativistic nuclear physics & Quantum
Chromodynamics”, JINR, Dubna, Russia 
Proceedings of XIX International Baldin Seminar on High Energy Physics Problems,
RELATIVISTIC NUCLEAR PHYSICS AND QUANTUM CHROMODYNAMICS , N XVI, V1
A.N.Skachkova. “Stop pair production in e+e - collisions at ILC”

39. E- spectra of quarks from W
STOP
TOP
Polarization : +- / -+
Polarization : ++/ --
QUARKS’2010 A.N.Skachkova. “Stop pair production in polarized γγ collisions at ILC”

40. PT- spectra of quarks from W
STOP
TOP
Polarization : ++/ --
Polarization : +- / -+
QUARKS’2010 A.N.Skachkova. “Stop pair production in polarized γγ collisions at ILC”

41. E- spectra of b-quarks STOP TOP Polarization : +- / -+
QUARKS’2010 A.N.Skachkova. “Stop pair production in polarized γγ collisions at ILC”

42. PT- spectra of b-quarks
STOP
TOP
Polarization : +- / -+
Polarization : ++/ --
QUARKS’2010 A.N.Skachkova. “Stop pair production in polarized γγ collisions at ILC”

43. θ of b-quarks for stop production
Polarization : +- / -+
Polarization : ++/ --
QUARKS’2010 A.N.Skachkova. “Stop pair production in polarized γγ collisions at ILC”